Wildlife Disease: Contagious Critters
- May 1, 2001
- Deworming & Internal Parasites
- Eastern Equine Encephalitis (EEE)
- Equine Protozoal Myeloencephalitis (EPM)
- Venezuelan Equine Encephalitis (VEE)
- West Nile Virus (WNV)
- Western Equine Encephalitis (WEE)
- Other Skin Problems
- Lyme Disease
- Pigeon Fever & Dryland Distemper
- Vesicular Stomatitis
- Rodent & Pest Control
Diseases from other animals pose a constant threat to our horses. Disease-causing agents, or pathogens, lurk in local wildlife, fly overhead in birds, and lay in the next field inside cows peacefully chewing their cuds. These disease agents--whether fungi, bacteria, or viruses--are just waiting for the opportunity to cross over into a horse. The diseases vary in circumstance, severity, and methods of transmission. We turned to John Timoney, MVB, PhD, DSc, MRCVS, Keeneland Professor of Infectious Diseases at the Maxwell H. Gluck Equine Research Center in Lexington, Ky., to help explain the role other animals play in equine infectious disease.
How often do horses catch diseases from wild or domestic animals? Timoney says it depends on the conditions. "I don't think you could come up with a percentage because these diseases are opportunistic." Outbreaks happen when the conditions are right, not just because the pathogens are present, which makes occurrences hard to predict.
What Affects Disease Transmission?
Geography plays a big part. For example, pseudotuberculosis is currently a problem in Oklahoma and Texas, says Timoney, "but we rarely see it here in Kentucky, and there are parts of the U.S. where you would never know it existed. So, it depends on what part of the country and even the time of year."
Weather can also affect transmission of diseases from other animals to horses. "If they were living in places where there was access to wildlife and perhaps a rainy season, then leptospirosis would be a concern," Timoney says. "In very dry areas, probably leptospirosis wouldn't be, but you'd have to worry about an organism like Corynebacterium pseudotuberculosis (which causes dryland distemper), which is associated more with arid parts of the U.S."
In horses, the disease caused by C. pseudotuberculosis is known as dryland distemper, dryland strangles, or pigeon fever. Despite the name, pigeon fever has nothing to do with birds. This disease is common worldwide in sheep and goats, but the bacterium can infect cattle and other animals. An infected horse develops distinctive swellings on the chest, giving a puffed-up, pigeon-esque appearance. (For more on dryland distemper, see the Up Front section of the December 2000 issue of The Horse; online at http://www.thehorse.com/ViewArticle.aspx?ID=1031.)
Direct and indirect contact influence a horse's risk of getting a disease from another animal. Timoney refers to this as opportunity for transmission. An example would be a pasture with a group of trees that have low-hanging branches where several types of animals congregate to shelter. Cattle, sheep, or even wild animals might be present some of the time, then horses are rotated into the field and might congregate there also.
Concentration of animals--and therefore of pathogens--affects the rate of disease transmission. "It is in roosting places where you are going to get concentrations of the particular pathogen," notes Timoney. "It doesn't matter as much out in the wild, because it is so spread out. It is said that the solution to pollution is dilution. That means the fewer organisms and the more they are spread out, the less the chance of infection. When there are large numbers of disease agents, a disease will spread more quickly."
Consider how much faster a human disease spreads through a city than a rural population. The same thing happens when wildlife congregates in or near a barn. "You provide more opportunities for transmission," says Timoney.
A barn is like a city for your horses. It not only provides a gathering place for animals and thus for pathogens, but forces more pathogens to collect in one place. Timoney said it is not just exposure to a disease-causing agent that causes a problem, but being exposed to a pathogen in numbers that will overwhelm the resistance or the immunity of that animal.
"It is the dose effect as much as opportunity for transmission, although they obviously are both linked together," says Timoney.
Timoney did have some good news: Most of the diseases in animals are host specific.
"The E. coli (bacteria) that cause disease in pigs are not found in other hosts because the E. coli have adapted very specifically to the pig. That's true of many of the disease agents. In the case of Streptococcus equi (which causes strangles) in the horse, it doesn't affect other hosts. It is very, very specific for equids."
That rule holds true in most animals--the pathogens in a particular animal have become adapted to that host and are not usually infectious for another host. Timoney said there are exceptions, such as anthrax, but the exceptions are rare.
One reason for a pathogen to be host-specific has to do with survival advantage for the disease agent. By specializing in one species of animal, the pathogen becomes as efficient as possible at taking advantage of that particular species' system. This would be difficult to achieve with a broad range of hosts. As with any other organism, patho-gens seek situations with optimal survival and propagation possibilities.
But Timoney warns that there are serious equine diseases that originate with other animals. "Sarcocystis neurona causes equine protozoal myeloencephalitis (EPM), which might not kill a horse, but the horse could be useless after the disease has progressed," he continues. "Salmonella can kill. Tyzzer's disease is a fatal disease in foals caused by Bacillus piliformis. So, it varies--some are quite lethal, and others more benign."
He mentioned that there are several cross-species pathogens that affect the skin and the integument (all external coverings, such as hair and hooves).
Viral Bad Boys
Perhaps some of the most serious diseases transmitted from other animals are the viral encephalitides: Venezuelan, Eastern, and Western equine encephalomyelitis (VEE, EEE, WEE) and West Nile virus. These diseases are present in a population of birds (the reservoir) and might or might not cause a health problem in the bird. Since the virus is circulating in the bird's bloodstream, when a mosquito (the vector or transmitter of the disease) bites a bird, it picks up the virus. Then, if the infected mosquito bites a horse, it deposits the virus. For the encephalitides, the horse is usually a dead-end host. This means that while the horse might develop the disease, it is unlikely to propagate the virus. Mosquitos can also vector VEE, EEE, and WEE from rodents, and the virus can live in mosquito populations until passed to a warm-blooded host.
Horses also can pick up diseases from birds that share their housing. "The classic diseases there are salmonella and Cryptococcus neoformans (the cause of cryptococcosis), which is a fungus that grows in bird feces," says Timoney. Cryptococcus neoformans is associated with wild birds, including pigeons, and with domestic chickens. Feces also figure in the transmission of histoplasmosis or salmonella from domestic birds, and Tyzzer's disease from rodent feces.
Dogs and cats are low on the list of dangers to horses. Rabies is an example of a problem that could be spread from a dog or cat to a horse, but that would be very unusual, says Timoney. "I suppose there is the potential for leptospirosis being transmitted from dogs, but it is unlikely. Cats don't have leptospirosis very much. All the diseases I know of in the cat are pretty well specific to the cat."
How It Gets From There To Here
Diseases are transmitted from other animals to horses directly and/or indirectly. Direct transmission could be a bite from a rabid animal, for example. Indirect transmission could be a mosquito spreading VEE. Other diseases that travel through a vector are Lyme disease from rodents via ticks; pseudotuberculosis from sheep and goats via biting insects; and vesicular stomatitis (VS) from cattle and swine via black flies and other biting insects. VS can also be spread through contaminated equipment.
Timoney explains that some diseases spread several ways. "In the case of ringworm, it would be contact or indirect contact. If a calf with ringworm had lived in that stable or rubbed against a fence and deposited the conidia (vegetative infectious spores) of the ringworm fungus, then a horse passing that way and rubbing there would pick it up."
An inanimate object that passes a disease is called a fomite. Fomites for ringworm can be brushes, blankets and tack, as well as fence posts. Ringworm can also come from rats and mice.
Leptospirosis, on the other hand, transmits through water. "Leptospires are water-based organisms that are adapted to live in the liquid phase. So they don't survive very long once they dry." Horses pick up this disease from places such as a pond contaminated with the urine from infected animals.
Another disease with an aqueous phase is Dermatophilus congolensis or rain rot (or rain scald). "Dermatophilus is one that's very interesting," Timoney says. "You've got a zoospore that gets into little rain droplets on leaves on branches. Then when another animal comes along, particularly if its back has been soaked and its skin is soft because of the water, the zoospore can start to work its way in through the skin and cause a lesion."
As in any battle, the first step is to know the enemy. What diseases are common in your area? Has the weather been disease-friendly lately? In your specific case, what animals does your horse come in contact with? You might not have cows, yet you ship to your trainer's farm, who has a few. Are you in a boarding barn in the suburbs, or are you deep in the woods by yourself? If you winter in a different state, the answers will change. For instance, your home state might have a rabies epidemic, and therefore your veterinarian will strongly recommend a rabies vaccination. However, when you get to the state where you will winter or compete, the veterinarian might not be as alarmed and could leave it to your discretion.
The second step is to know the enemy's plan of attack, then interrupt it. "Starting with leptospirosis," says Timoney, "if the horses are sharing an environment with wildlife and you know that you have raccoons and opossums, then you should fence off areas of standing water where these wildlife would go to drink. That's hard to do in some places where you have a stream. You can't fence everything, but that is the way by which it gets transmitted, so the extent that you can minimize the access of the horse to places where the wild animal is going to be, you will reduce the chance of transmission."
Prevention for other diseases involves breaking their life cycles. For example, keep a clean barn and a secure feed system to prevent contamination by feces, and thus transmission of diseases present in feces such as salmonella, Cryptococcus neoformans, and EPM. Also, keep disease-carrying animals and insects from taking up residence in the barn. To avoid diseases transmitted by mosquitoes, remove any standing water that could be a breeding site; for example, that old barrel down by the junk shed that's half-full of water.
Good husbandry is always good medicine--clean, healthy horses in healthy environments are less susceptible to disease. Avoiding stress on your horses is also a big help. Stressors for horses are the same things that stress people--hard work, sickness, travel, etc. Salmonella in particular is associated with stress. Antibiotics, which disturb the intestinal flora, and the stress of visiting a vet clinic can also leave a horse susceptible to salmonella.
Vaccines exist to fight many of these diseases. In the case of diseases that do not have a vaccine, stay tuned. Many researchers around the world are striving to find answers. Timoney's lab, for example, is working on a vaccine for leptospirosis and strangles.
Finally, Timoney reminds us that these cross-species diseases, while serious, play a small part in equine infectious diseases. The outbreaks of diseases transmitted from other animals are sporadic and have a limited effect on the overall horse population; however, it is always good to be aware of what can be transmitted from critter to critter and take measures to prevent this.
|DISEASE||TRANSMISSION TO HORSES||RESERVOIR|
|Anthrax||grazing on infected pasture||almost all animals (rare in the horse)|
|Cryptococcosis||fecal contamination||birds (rare)|
|Equine Protozoal Myeloencephalitis (EPM)||fecal contamination||opossums, possibly more|
|Histoplasmosis||fecal contamination||birds, bats (rare)|
|Pigeon Fever (Dryland Distemper)||insect vector||sheep, goats|
|Potomac Horse Fever||unknown--pasture?/Caddis flies?/snails?||unknown vertebrate host|
|Rabies||bites from infected animals||warmblooded animals (rare)|
|Rain rot or Rain scald||contaminated water||cattle|
|Ringworm||contact, indirect contamination||cattle, rodents|
|Tuberculosis||fecal contamination||wild birds (rare)|
|Tyzzer's Disease||fecal contamination||rodents|
|Vesicular stomatitis||insect vector, equipment||cattle, swine|
|WEE, VEE, EEE||insect vector||birds and rodents|
|West Nile virus||insect vector||birds|
About the Author
Katherine Walcott is a freelance writer living in the countryside near Birmingham, Al. She writes for anyone she can talk into paying her and rides whatever disciplines she can talk her horses into doing.